4 Answers
4

Interrupts Target’s Action: An opportunity action takes place before the target finishes its action. After the opportunity attack, the creature resumes its action. If the target is reduced to 0 hit points or fewer by the opportunity attack, it can’t finish its action because it’s dead or dying.

A normal OA will not interrupt movement. Even if the creature is hit, it can continue it's movement as normal.

That said, it is possible to stop the movement, but you'd need a power or ability that explicitly says so. For example, the stance Defend the Line would allow the fighter to slow the enemy on an OA. The creature would then only be able to move a total of 2 squares for his current move action. If he has already moved 2 or more squares, he stops in the square he provoked the OA from.

There's also the fighter's Combat Superiority which stops movement altogether.

Combat Superiority

You gain a bonus to opportunity attacks equal to your Wisdom modifier. An enemy struck by your opportunity attack stops moving, if a move provoked the attack. If it still has actions remaining, it can use them to resume moving.

Sure. By the time I get mine in, someone's done it better. :)
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Allen GouldJun 22 '11 at 16:58

Agreed on this. Think about a Running Back. The character knows that they are breaking an engagement with a deadly threat, and they have plans to break out. The threat is going to try to stop them, but if they are still on their feet they are going to push through the pain and keep going. If they are not dead or disabled, they will smash through the line!
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WayneDenierJun 23 '11 at 4:13

Here's a question for clarification. Do you treat each square of movement as though it were were a separate miniature action? So you move X squares, get interrupted, and then move the remainder of your movement from wherever you landed, if possible?
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jpreteAug 19 '11 at 17:25

PHB1 on p290 goes into detail on opportunity attacks. As far as I read it, unless an attack that is being used for the opportunity attack specifically mentions that it stops the character, then they can continue moving.

For example, the fighter's Combat Superiority ability reads

An enemy struck by your opportunity attack stops moving, if a move provoked the attack. If it still has actions remaining, it can use them to resume moving.

Interrupts Target’s Action: An opportunity action takes place before the target finishes its action. After the opportunity attack, the creature resumes its action. If the target is reduced to 0 hit points or fewer by the opportunity attack, it can’t finish its action because it’s dead or dying.

Conceptually, the attack happens while they're moving - unless the attack does something to prevent the movement (dropping the target is listed, I'd also say adding the slowed or immobilized conditions would work as well), the target finishes the action after the response.

Indeed. It's interesting that the "immobilized" or "slowed" conditions wouldn't stop a ranged or area attack that provoked an OA. Of course, I left that kind of stuff out of the question :)
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Adriano Varoli PiazzaJun 22 '11 at 17:04

2

No, however Dazed would if the target had already taken a minor or move action during the turn. If no other actions had been taken dazed has no effect.
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wax eagleJun 22 '11 at 17:18

If the creature or PC is attacking and provokes an opportunity attack we roll the opportunity attack and if the roll is a Critical Hit we stop the movement and allow them to use the rest of their actions or even an Action Point to finish out their turn, if they have an action point.

This has worked for my group for awhile now and I have seen other groups use this method in Encounters as well.